    Patent Document 1: Japanese Laid-Open Patent Publication No. 10-224262
(Tags for Radio Frequency Identification Communication Systems)
RFID is wireless equipment for identifying mobile objects, which is attached to or held by a commodity, a person, a car, a road sign, etc., and in response to a radio wave from an interrogator (a reader/writer), notifies individual information and position information to the interrogator.
The RFID is considered not only as an alternative to the barcode, but also as elemental technology for implementing an entirely new infrastructure for a future network community. The research and development of RFID currently underway mainly concentrates on communication within a relatively short range of several tens of centimeters, but if it is possible to realize low-cost small RFID tags capable of communication from relatively afar, such as from about 10 meters away, the application range will be conceivably further extended.
When such is attached to a commodity, a person, a car or a road sign, for example, its individual information and position information are read by a mobile object from 10 m away, making it possible to readily enjoy safety and convenience.
The above Patent Document 1 describes a conventional passive RFID tag device. According to this, the conventional passive RFID tag device is basically configured as shown in FIG. 1, and therefore has a problem that its possible communication range is short for the following reasons.
(1) A response signal is created by frequently changing impedance Zv between antenna feeding points (1) and (2) to repeat reflection and absorption of incident radio waves, so that the transmission output (conversion efficiency) is low, and the load impedance Zv is applied between the feeding points (1) and (2), resulting in considerable loss of received power.
(2) The source voltage for a control circuit is created by directly diode-rectifying an RF signal received between the antenna feeding points (1) and (2), and therefore the output voltage is low.
(3) ASK or BPSK is used as a subcarrier modulation method, and therefore the amount of information that can be transmitted per transmission power is small.